1. Field of the Invention
The present invention relates to a lithium-air secondary battery.
2. Description of the Related Art
One candidate of innovative batteries is a metal-air battery. In metal-air batteries, oxygen involved in the battery reaction is supplied from air, thus making it possible to utilize an inner space of the battery cell for packing a negative electrode active material at the maximum, and thereby achieving principally a high energy density.
The majority of the metal-air batteries currently proposed falls into lithium-air batteries. In an ordinary lithium-air battery, O2 is reduced to produce Li2O at an air electrode (positive electrode) side during discharging while lithium is oxidized to produce Li+ at a negative electrode side as shown in the following reaction formulas below. Then, the reverse reactions occur during charging.Positive electrode: O2+4e−+4Li+→2Li2ONegative electrode: Li→Li++e−
For example, Patent Document 1 (JP 2010-176941A) discloses an lithium-air battery including a negative electrode comprising lithium metal, an electrolytic solution for the negative electrode, a solid electrolyte separator, through which lithium ions can pass exclusively, an electrolytic solution for an air electrode, and the air electrode, all of which are provided in this order. This document proposes using an organic electrolytic solution as the electrolytic solution for the negative electrode while using an alkaline aqueous electrolytic solution as the electrolytic solution for the air electrode. This configuration may cause a problem in that carbon dioxide in air passes through the air electrode and reacts with the alkaline aqueous electrolytic solution to produce an alkali metal carbonate, which deteriorates the electrolytic solution, and also may cause a problem in that the alkali metal carbonate blocks pores in the air electrode, and is thus not suitable for long-term use.
Patent Document 2 (WO2009/104570) discloses a suppression of the carbonate precipitation caused by carbon dioxide as described above by providing an anion-exchange polymer membrane at the interface between the air electrode and the alkaline electrolytic solution in metal-air batteries or alkaline fuel cells which utilize an alkaline aqueous electrolytic solution. In such a configuration, OH− ions are produced from oxygen and water on the catalyst surface in the air electrode, and the OH− ions move through the anion-exchange polymer membrane and the electrolytic solution to react with the metal negative electrode in the case of a metal-air battery, thus enabling battery operation. Deterioration of the air electrode performance is lower than the case without the anion-exchange polymer membrane, but the deterioration is not fully suppressed.
Non-Patent Document 1 (ECS Transactions, 28(32) 1-12 (2010)) discloses an aqueous lithium-air battery having a structure for preventing incorporation of carbon dioxide in air. In the structure, a film made of a lithium superionic conductor (LISICON) is used as a solid electrolyte separator to isolate a lithium metal negative electrode as in Patent Document 1, and an air electrode having a polymeric anion-exchange membrane is provided as in Patent Document 2. The use of an air electrode provided with an anion-exchange membrane according to Non-Patent Document 1 extends the battery life, but the problem caused by carbon dioxide is not fully solved. This is considered to be because the polymeric anion-exchange membrane cannot completely prevent penetration of carbon dioxide.
Meanwhile, recently, a layered double hydroxide (LDH) represented by the general formula M2+1-xM3+x(OH)2An−x/n.mH2O (wherein M2+ is a divalent cation, M3+ is a trivalent cation, and An− is an anion having a valency of n) is known as a hydroxide-ion conductive solid electrolyte. For example, Patent Document 3 (WO2010/109670) proposes the use of a film of a layered double hydroxide as an alkaline electrolyte film for a direct alcohol fuel cell. Besides the layered double hydroxides, Patent Document 4 (WO2011/108526) discloses the use of a hydroxide-ion conductive solid electrolyte layer mainly composed of NaCo2O4, LaFe3Sr3O10, Bi4Sr14Fe24O56, NaLaTiO4, RbLaNb2O7, KLaNb2O7, or Sr4Co1.6Ti1.4O8(OH)2.xH2O for a fuel cell.
On the other hand, as a solid electrolyte having lithium ion conductivity, a garnet-type ceramic material having a Li7La3Zr2O12 (hereinafter referred to as LLZ)-based composition has been attracting attention. For example, Patent Document 5 (JP 2011-051800A) discloses that addition of Al together with Li, La, and Zr, which are the fundamental elements of LLZ, can enhance denseness and lithium ion conductivity. Patent Document 6 (JP 2011-073962A) discloses that addition of Nb and/or Ta together with Li, La, and Zr, which are the fundamental elements of LLZ, can further enhance lithium ion conductivity. Patent Document 7 (JP 2011-073963A) discloses that denseness can be further enhanced by containing Li, La, Zr, and Al and bringing the molar ratio of Li to La to 2.0 to 2.5.
Regarding the alkaline electrolytic solution, Patent Document 8 (JP 2012-33490A) discloses a lithium-air battery in which the aqueous electrolyte contains lithium hydroxide and lithium halide, and describes that by including lithium halide in the aqueous electrolyte, the reaction between lithium hydroxide and a solid electrolyte film is suppressed, and thus the negative electrode can be protected.